Apparatus, system, and method of inspecting image, and recording medium storing image inspection control program

ABSTRACT

An inspection apparatus, inspection system, inspection method, and inspection control program stored in a recording medium, each of which sets a reference point in a master image and a read image read from a printed image, which is to be used for detecting the positional shift between the read image and the master image, based on determination whether a pattern previously added to the printed image for detecting the positional shift is available or can be used to effectively detect the positional shift.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/572,989, filed on Aug. 13, 2012, which claims priority to claimspriority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos.2011-178080, filed on Aug. 16, 2011, and 2012-162680, filed on Jul. 23,2012, in the Japan Patent Office, the entire disclosure of which ishereby incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to an apparatus, system, and method ofinspecting an image formed by an image forming apparatus, and arecording medium storing an image inspection control program.

2. Background

The recent image forming system may be provided with an inspectionapparatus, which reads a printed image output from an image formingapparatus, generates a master image from image data of the printedimage, and compares the read image with the master image to determinewhether the printer image sufficiently reproduces the image data. Tocompare between the read image and the master image, each pixel in theread image needs to be compared with a corresponding pixel in the masterimage. In view of this, the pixel position of the master image may becorrected so as to match the pixel position of the read image, which mayshrink in size or may be skewed during print processing, by enlarging orreducing the size of the master image or rotating the master image. Forexample, the inspection apparatus may specify a plurality of referencepoints in the read image, which correspond to a plurality of referencepoints in the master image, by pattern matching. Based on the positionalshifts of the reference points between the read image and the masterimage, a correction parameter to be used for correcting the master imagemay be set.

While the reference point is usually defined as a marker in case ofoffset printing, when cut paper is used for printing, the referencepoint needs to be defined to make comparison between the read image andthe master image. Japanese Patent Application Publication No.2004-195878 discloses a technique of adding a yellow dot pattern to animage to be formed on a recording sheet, and using the yellow dotpattern as the reference point to detect the positional shift in theprinted image with the master image. This technique of using the yellowdot pattern is applicable when the printed image is a color image thatis formed using a color image forming apparatus. When a monochrome imageforming apparatus is used or when a monochrome image is to be printed,the yellow dot pattern is not added such that it would not be possibleto inspect the printed image using the reference point.

Further, when the yellow dot pattern is added to an area of the printedimage having yellowish color, it may be difficult for the inspectionapparatus to extract the yellow dot pattern from the read image. In suchcase, Japanese Patent Application Publication No. 2004-195878 uses aprojection histogram generated for each one of the vertical andhorizontal directions of the image. However, when the printed image isskewed, such technique using the histogram cannot be used to correctlydetect the positional shift in the printed image.

SUMMARY

In view of the above, one aspect of the present invention is to providean apparatus, system, method, and an inspection control program storedin a recording medium, each of which is capable of setting a referencepoint, respectively, in a master image and a read image read from aprinted image, which is to be used for detecting the positional shiftbetween the read image and the master image, based on determinationwhether a pattern previously added to the printed image for detectingthe positional shift is available or can be used to effectively detectthe positional shift.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages and features thereof can be readily obtained and understoodfrom the following detailed description with reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic block diagram illustrating an image forming systemincluding an inspection apparatus, according to an example embodiment ofthe present invention;

FIG. 2 is a schematic block diagram illustrating a hardware structure ofa control section of any one of an engine controller, a print engine,and the inspection apparatus of FIG. 1, according to an exampleembodiment of the present invention;

FIG. 3 is a schematic block diagram illustrating functional structuresof the print engine and the inspection apparatus of FIG. 1, according toan example embodiment of the present invention;

FIG. 4 is an illustration for explaining a P pattern to be added to animage, according to an example embodiment of the present invention;

FIG. 5 is an illustration for explaining the image to which the Ppattern of FIG. 4 is added;

FIG. 6 is a flowchart illustrating operation of generating a masterimage, performed by the inspection apparatus of FIG. 1, according to anexample embodiment of the present invention;

FIG. 7 is an example data structure of a P pattern availability table,managed by inspection apparatus of FIG. 1;

FIG. 8 is a flowchart illustrating operation of setting a referencepoint in the image, performed by the inspection apparatus of FIG. 1,according to an example embodiment of the present invention;

FIG. 9 is an illustration of an example edge extraction filter to beapplied to the image to generate an edge image;

FIGS. 10A to 10C are an illustration for explaining example operation ofextracting corners of the image as a reference point;

FIGS. 11A to 11D are an illustration for explaining a corner extractionfilter used for extracting the corners of the image;

FIG. 12 is an illustration for explaining example operation ofsegmenting the image into a plurality of image areas;

FIG. 13 is an example data structure of a corner coordinate table,managed by the inspection apparatus of FIG. 1, according to an exampleembodiment of the present invention;

FIG. 14 is an example data structure of a reference point selectionresult table, managed by the inspection apparatus of FIG. 1;

FIG. 15 is a flowchart illustrating operation of inspecting a printedimage, performed by the inspection apparatus of FIG. 1, according to anexample embodiment of the present invention;

FIG. 16 is a flowchart illustrating operation of correcting a materimage, performed by the inspection apparatus of FIG. 1, according to anexample embodiment of the present invention;

FIG. 17A is a schematic block diagram illustrating an image formingsystem including an inspection apparatus, according to an exampleembodiment of the present invention;

FIG. 17B is a schematic block diagram illustrating an image formingsystem including an inspection apparatus, according to an exampleembodiment of the present invention; and

FIG. 18 a schematic block diagram illustrating an image forming systemincluding an inspection apparatus, according to an example embodiment ofthe present invention.

The accompanying drawings are intended to depict example embodiments ofthe present invention and should not be interpreted to limit the scopethereof. The accompanying drawings are not to be considered as drawn toscale unless explicitly noted.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“includes” and/or “including”, when used in this specification, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

In describing example embodiments shown in the drawings, specificterminology is employed for the sake of clarity. However, the presentdisclosure is not intended to be limited to the specific terminology soselected and it is to be understood that each specific element includesall technical equivalents that operate in a similar manner.

In the following examples, an image forming system is provided, whichincludes a digital front end (DFE) that generates a binary image ofimage data to be printed using a dither matrix, a print engine thatforms a printed image based on the binary image under control of anengine controller, and an inspection apparatus that inspects the printedimage formed by the print engine 3 using a master image that isgenerated by converting the binary image into a multivalue image.

FIG. 1 illustrates a configuration of an image forming system accordingto an example embodiment of the present invention. The image formingsystem of FIG. 1 includes a digital front end (DFE) 1, an enginecontroller 2, a print engine 3, and an inspection apparatus 4. The imageforming system of FIG. 1 may be used for production printing in which alarge number of images are sequentially printed and output. The imagesto be printed may be the same or different, depending on the image data.

The DFE 1 receives a print job, which includes image data to be printed,from an information processing apparatus such as a personal computerthrough a network. The DFE 1 applies halftone processing to image datato be printed to generate a binary image, and outputs the binary imageand dither matrix data that is used for halftone processing to theengine controller 2. The binary image may be formed as an image in whicheach image pixel is expressed in chromatic or achromatic color (such asblack or white). Alternatively, the binary image may be formed as animage in which each image pixel is expressed in two of cyan, magenta,yellow, and black colors.

The engine controller 2 receives the binary image and the dither matrixdata from the DFE 1, and controls the print engine 3 to form an imagebased on the binary image. The engine controller 2 further inputs thebinary image to the inspection apparatus 4.

The print engine 3 forms an image on a recording sheet based on thebinary image under control of the engine controller 2, and outputs theformed image as a printed image. The print engine 3 further reads theprinted image using a reading device to generate read image data (“readimage”), and inputs the read image to the inspection apparatus 4.

The inspection apparatus 4 converts the binary image input from theengine controller 2 into a multivalue image to generate a master image,which is used for inspecting the printed image of the print engine 3.The inspection apparatus 4 compares a reference point in the read imageinput from the print engine 3 with a reference point in the master imageto correct pixel positions of the read image or the master image beforeinspecting the printed image of the print engine 3. More specifically,based on the difference in the reference point, the inspection apparatus4 corrects the pixel positions of the read image or the mater image,such that the positional shift in the read image is corrected. Theinspection apparatus 4 further inspects the read image that reflects theprinted image, based on difference between the read image and the masterimage in pixel value.

Referring now to FIG. 2, a hardware structure of the inspectionapparatus 4 is explained according to an example embodiment of thepresent invention.

As illustrated in FIG. 2, the inspection apparatus 4 is implemented byan information processing apparatus such as a personal computer or aserver computer. The inspection apparatus 4 includes a centralprocessing unit (CPU) 10, a random access memory (RAM) 20, a read onlymemory (ROM) 30, a hard disk drive (HDD) 40, and an interface (I/F) 50,which are connected through a bus 90. The inspection apparatus 4 furtherincludes a liquid crystal display (LCD) 60, an operation device 70, anda specialized device 80, which are connected to the I/F 50.

The CPU 10 is implemented by a processor such as a microprocessor, whichis capable of controlling entire operation of the inspection apparatus4. The RAM 20 is implemented by a volatile memory that writes variousdata thereto or reads various data therefrom with relatively highspeeds. The RAM 20 may be used as a work memory area of the CPU 10. TheROM 30 is implemented by a nonvolatile memory from which various data isread. The ROM 30 may store various programs such as firmware. The HDD 40is implemented by a nonvolatile memory from which various data is read.The HDD 40 may store various control programs such as an operatingsystem (OS), and application programs such as the inspection controlprogram.

The I/F 50 allows various hardware devices to be connected through thebus 90 or to the outside through a network, and controls theseconnections. The LCD 60 functions as a user interface, which allows auser to visually check status of the inspection apparatus 4. Theoperation device 70 functions as a user interface, which allows the userto input various data to the inspection apparatus 4 using, for example,a keyboard or a mouse. The LCD 60 and the operation device 70 may beintegrated into one device, for example, in the form of a touch panelscreen.

The specialized device 80 is a hardware device that causes theinformation processing apparatus of FIG. 2 to additionally havespecialized functions to cause the information processing apparatus tofunction as the inspection apparatus 4. More specifically, with thespecialized device 80, the CPU 10 converts the binary image into themultivalue image to generate the master image, adding a pattern to themaster image, or compares the master image with the read image of theprinted image. The specialized device 80 may be implemented by, forexample, Application Specific Integrated Circuit (ASIC).

The specialized functions of the inspection apparatus 4 may bealternatively implemented by software, such as the inspection controlprogram that is stored in a memory such as the ROM 30, HDD 40, or anydesired recording medium such as an optical disc. When executed by theCPU 10, the inspection control program may be read onto the RAM 20 tocause the CPU 10 to control various hardware devices of FIG. 2 accordingto the control program.

Alternatively, the specialized functions of the inspection apparatus 4may be realized by a combination of software and hardware such as acombination of the inspection control program and the ASIC of thespecialized device 80.

The control section of the print engine 3 is substantially similar inhardware structure to the inspection apparatus 4 of FIG. 2, except thatthe specialized device 80. The specialized device 80 causes theinformation processing apparatus of FIG. 2 to additionally havespecialized functions such that the information processing apparatusfunctions as the print engine 3. More specifically, the specializeddevice 80 of the print engine 3 includes a plotter that forms a printedimage on a recording sheet, and a reading device that reads the printedimage into read image data.

The control section of the engine controller 2 is substantially similarin hardware structure to the inspection apparatus 4 of FIG. 2, exceptfor the specialized device 80. The specialized device 80 causes theinformation processing apparatus of FIG. 2 to additionally havespecialized functions such that the information processing apparatusfunctions as the engine controller 2. For example, the specializeddevice 80 causes the engine controller 2 to control forming of a printedimage and inspecting of the printed image.

FIG. 3 illustrates a schematic block diagram illustrating functionalstructures of the print engine 3 and the inspection apparatus 4,according to an example embodiment of the present invention. Asillustrated in FIG. 3. the print engine 3 includes a print processor301, a reading device 302, and a P pattern generator 303. The inspectionapparatus 4 includes a read image obtainer 401, a master image generator402, a P pattern generator 403, an inspection controller 404, and acomparator 405.

The print processor 301 obtains the binary image from the enginecontroller 2, and forms an image of the binary image on a recordingsheet to output a printed image. In this example, the print processor301 is implemented by an image forming device that forms an image usingthe electrophotographic method, such as a tandem-type image formingdevice. Alternatively, the print processor 301 may be implemented by anyother desired image forming device such as an inkjet printer. In thisexample, the binary image generated by the engine controller 2 is animage in which each pixel is expressed by 1 bit of cyan, magenta,yellow, and black colors (total of 4 bit), with resolution of 600 dotsper inch (dpi). The engine controller 2 further inputs the binary imageto the inspection apparatus 4.

The reading device 302 reads the printed image formed on the recordingsheet, which is output from the print processor 301, into read imagedata, and outputs the read image to the inspection apparatus 4. Thereading device 302 is implemented by a line scanner, which is providedin the print engine 3 such that the reading device 302 can scan theprinted image formed on the recording sheet as the recording sheet istransferred and output from the print engine 3. For example, the readingdevice 302 may be disposed along a transfer passage through which therecording sheet is transferred. As the recording sheet is beingtransferred, the reading device 302 reads the printed image formed onthe recording sheet by scanning the surface of the recording sheet.

The P pattern generator 303 generates a P pattern, which is a yellowcolor pattern that is hardly perceptible to the human eye, and inputsthe P pattern to the print processor 301. When forming the image on therecording sheet, the print processor 301 causes the P pattern to besuperimposed on a layer of the printed image such that the printed imageand the P pattern are formed on the recording sheet.

FIG. 4 illustrates one unit of an example P pattern. The P patternincludes 64 dots in the vertical direction and 64 dots in the horizontaldirection. Each circle in FIG. 4 has a size that is 2 dots in thevertical direction and 2 dots in the horizontal direction. In thisexample, the P pattern contains information indicating a specificapparatus that outputs the printed image formed on the recording sheet.More specifically, the P pattern is generated according to a code thatis uniquely assigned to each apparatus, such that a specific apparatusthat outputs the printed image can be identified using the P pattern.

The black circle in FIG. 4 indicates dots functioning as a mark fordefining one unit of P pattern, i.e., an area specifying one unit of Ppattern. The dots expressed in black circle are to be added,irrespective of the contents of a code. In this example, the dotexpressed in black color is added to the upper left corner of each unitof P pattern. The white circle in FIG. 4 indicates dots functioning as amark for defining one unit of P pattern, but is not formed irrespectiveof the contents of the code. In this example, the dots expressed inwhite color are added at edges of one unit of P pattern. The gray circlein FIG. 4 indicates dots functioning as a mark for specifying thespecific apparatus that forms the printed image, and is to be addeddepending on the contents of the code. With the dots expressed in graycolor, a specific apparatus forming the printed image can be identified.Further, it is to be noted that each dot of the P pattern of FIG. 4,which is to be added to the printed image, is formed with yellow color.

FIG. 5 illustrates a part of the recording sheet formed with the printedimage and the P pattern added to the printed image. Each of the grayboxes in FIG. 5 indicates one unit of the P pattern of FIG. 4. Asillustrated in FIG. 5, a plurality of units of P pattern are arranged inzigzag from the edge of an area where the image is printed (“printarea”).

In this example, the master image to be generated by the inspectionapparatus 4 is added with a plurality of units of P pattern of FIG. 4 ina form indicated by FIG. 5. The P patterns, which may be detectedrespectively in the read image and the master image, are later used as areference point to match the pixel positions of the read image and themaster image. Some units of the P pattern, when overlapped with theprinted image formed on the recording sheet, can be hardly detected.Especially when the image has a section with a yellowish backgroundcolor, the units of yellow P pattern that are added to that section arehardly detected such that these units of P pattern may not beeffectively used as the reference point. In view of this, in thefollowing examples, the inspection apparatus 4 further extracts cornersof the image, as a candidate reference point. The inspection apparatus 4selects one or more reference points that can be effectively used todetect the positional shift from the P pattern units and the corners,thus allowing comparison between the read image and the master image beperformed with improved accuracy.

Further, when the printed image is generated in monochrome, the Ppattern is not added to the printed image to be formed on the recordingsheet. In such case, the inspection apparatus 4 uses the corners of theimage, which are extracted respectively from the master image and theread image, as a reference point to detect the positional shift in theread image with respect to the master image.

Referring back to FIG. 3, a functional structure of the inspectionapparatus 4 is explained according to an example embodiment of thepresent invention. The read image obtainer 401 obtains the read imageoutput from the print engine 3, and inputs the read image to thecomparator 404 as an image for inspection. The master image generator402 obtains the binary image input by the engine controller 2, andgenerates a master image to be compared with the read image subjectedfor inspection. The master image generator 402 sets a reference point inthe master image, which is used to match the pixel positions of the readimage and the master image.

The P pattern generator 403 is substantially similar in function to theP pattern generator 303 of the print engine 3. More specifically, the Ppattern generator 403 generates the P pattern as illustrated in FIG. 4,and inputs the P pattern to the master image generator 402. With imagedata of the P pattern, the master image generator 402 generates themaster image to which the P pattern is added in a substantially similarmanner as described above referring to FIG. 5.

The inspection controller 404 functions as a controller that controlsentire operation of the inspection apparatus 4, for example, bycontrolling each unit or device of the inspection apparatus 4. Thecomparator 405 compares the read image input by the read image obtainer401 with the master image generated by the master image generator 402 todetermine whether the printed image formed by the print engine 3 is ingood quality as expected. The comparator 404 may be implemented by theASIC of the specialized device 80 such that the comparator 404 is ableto compute a large amount of data with high speeds.

Referring now to FIG. 6, operation of generating a master image,performed by the master image generator 402, is explained according toan example embodiment of the present invention.

At S601, the master image generator 402 obtains the binary image fromthe engine controller 2, and determines whether the binary image iscolor (chromatic) or monochrome. In alternative to determining whetherthe binary image is color or monochrome by referring to the binaryimage, the master image generator 402 may determine whether the binaryimage is color or monochrome based on flag information indicatingwhether the binary image is color or monochrome. The flag informationmay be generated by the engine controller 2, and sent to the inspectionapparatus 4 together with the binary image.

When it is determined that the binary image is color (“YES” at S601),the master image generator 402 determines that the P pattern is to beadded to the binary image obtained from the engine controller 2 tooutput a determination result. Based on the determination result, theinspection controller 404 instructs the P pattern generator 403 togenerate image data of P pattern. At S602, the master image generator402 obtains image data of the P pattern from the P pattern generator 403under control of the inspection controller 404, and combines the binaryimage with the P pattern by superimposing the P pattern on the binaryimage, in a substantially similar manner as described above referring toFIG. 5.

Further, at S602, as illustrated in FIG. 5, the master image generator402 assigns an identification number (such as “#1, 1”, “#1, 2”, etc.) toeach unit of the units of the P pattern that are added to the masterimage. Each of the identification number of the P pattern unit ismanaged in association with location information indicating a specificcoordinate value of the master image at which the specific unit of Ppattern is formed. In this manner, the identification number can be usedto specify a specific P pattern unit in the master image.

The master image generator 402 further determines whether each one ofthe P pattern units can be used as a reference point. Assuming that themaster image generator 402 obtains the binary image in which each pixelis expressed in K and Y colors, for the K plane and the Y plane of thebinary image, the master image generator 402 determines whether an areato which the P pattern, assigned with a specific identification number,is added contains any image data to generate a determination result.Based on the determination result, the master image generator 402generates a P pattern availability table indicating whether the Ppattern assigned with the specific identification number can be used asa reference point for detecting the positional shift in the read image.For example, the P pattern availability table of FIG. 7 may be generatedand stored in any desired memory of the inspection apparatus 4.

Referring to FIG. 7, the P pattern availability table stores, for eachone of the P pattern units of the master image, a pattern number that isone example of an identification number, a coordinate value indicatingthe coordinate value of the master image to which the P pattern unit isadded, and availability information indicating whether the P patternunit can be used as a reference point, in association with one another.

In this example, the master image generator 402 determines that the Ppattern unit can be used as a reference point, when an area to which theP pattern unit having the specific identification number is added, hasno image data for both of the K and Y planes. More specifically, whenthe area to which the P pattern is added contains no image data, themaster image generator 402 enters the value “Y” indicating that the Ppattern unit can be used as a reference point, in association with thepattern number and the coordinate value of the P pattern availabilitytable of FIG. 7. When the area to which the P pattern is added containsimage data, the master image generator 402 enters the value “N”indicating that the P pattern unit is not appropriate as a referencepoint. Alternatively, the mater image generator 402 may only enter the Ppattern unit that can be used as a reference point in the table, suchthat the table of FIG. 7 may only store the P pattern units that aredetermined as a candidate for a reference point. In such case, a fieldfor the availability information may not be provided in the table.

Further, in this example, as illustrated in FIG. 5, the coordinate valueof the P pattern availability table of FIG. 7 is a coordinate value ofthe upper left corner of the square shape of the P pattern. The leftcorner of the P pattern corresponds to the dot expressed in blackcircle, which is located at the upper left of the P pattern of FIG. 4.

Referring back to FIG. 6, at S603, the master image generator 402converts the binary image in which each pixel is expressed in one bit ofKY colors, to a multivalue image in which each pixel is expressed ineight bits of CMYK colors. In this example, resolution of the multivalueimage is 600 dpi.

At S604, the master image generator 402 further converts the multivalueimage of 600 dpi to the multivalue image of 200 dpi. Along withresolution conversion, the coordinate value is multiplied or dividedaccording to the degree of change in resolution. For example, in case ofgenerating the master image of 200 dpi based on the binary image of 600dpi, the coordinate value of the binary image, obtained at S602, isdivided by 3 to obtain the coordinate value of the master image withlower resolution. This coordinate value of the master image is thenstored in the P pattern availability table of FIG. 7.

At S605, the master image generator 402 converts the 8-bit CMYK image toa 24-bit RGB image, with resolution of 200 dpi. In this manner, themultivalue image, i.e., the master image, has a data format that isequal to the data format of the read image generated by the printprocessor 3.

When it is determined that the binary image is monochrome (“NO” atS601), the operation proceeds to S603, without performing S602. SinceS602 is not performed, the P pattern is not added to the master image.In such case, at S603, the master image generator 402 converts thebinary image of 1 bit to the binary image of 8 bits. At 605, the masterimage generator 402 converts a K component of the binary image to a RGBimage to generate a master image.

At S606, the master image generator 402 sets a reference point in themaster image, which is used to match the pixel positions of the masterimage and the read image.

Referring now to FIG. 8, operation of setting a reference point in themaster image, performed by the master image generator 402 at S606, isexplained according to an example embodiment of the present invention.

At S801, the master image generator 402 generates an edge image based onthe master image. More specifically, the master image generator 402applies an edge extraction filter as illustrated in FIG. 9 to the masterimage to extract an edge in the master image. The edge extraction filterof FIG. 9 is referred to as a Laplasian filter, which extracts the edgein the image based on the difference in image pixel between the adjacentpixels. Assuming that the edge extraction filter of FIG. 9 is applied toan image of FIG. 10A, the edge of the image is extracted as illustratedin FIG. 10B. In FIG. 10A, an area shown by diagonal lines is assumed tobe a solid image area.

In alternative to applying the edge extraction filter of FIG. 9, theedge image may be extracted using any desired technique.

At S802, the master image generator 402 applies a plurality of filtersto the edge image extracted at S801 to extract the corners of the edgeimage. For example, by applying corner extraction filters of FIGS. 11Ato 11D to the edge image of FIG. 10B, four corners of the edge image areextracted as illustrated in FIG. 10C. The corner extraction filters ofFIG. 11A to 11D each extract the corner in the image based on thedifference in image pixel between the adjacent pixels.

More specifically, the master image generator 402 segments the maserimage into a plurality of areas, as illustrated in FIG. 12. The area Ais applied with the corner extraction filter of FIG. 11A to extract theupper left corner of the image. The area B is applied with the cornerextraction filter of FIG. 11B to extract the upper right corner of theimage. The area C is applied with the corner extraction filter of FIG.11C to extract the lower left corner of the image. The area D is appliedwith the corner extraction filter of FIG. 11D to extract the lower rightcorner of the image. In this manner, four corners of the master imageillustrated in FIG. 10C can be extracted from the edge image of FIG.10B.

In the above-described example, four corners of the master image areextracted using the corner extraction filters of FIG. 11A to 11D.Alternatively, any desired technique may be applied to extract thecorners of the master image.

For example, each area of the master image may be applied with thecorner extraction filters of FIG. 11A to 11D to extract the corners ofthe master image. By applying a specific corner extraction filter foreach area, the processing load required for corner extraction can bereduced.

Further, in alternative to extracting the corner after extracting theedge image, the corner extraction filters may be applied to the masterimage of FIG. 10A. By applying the corner extraction filter to only theedge image of FIG. 10B, however, the processing load required for cornerextraction can be greatly reduced.

When the corners of the master image are specified, the master imagegenerator 402 assigns an identification number (“coordinate number”) tothe coordinate value at which each corner is located, and stores thecoordinate number and the coordinate value for each one of the extractedcorners of the master image in association with one another to generatea corner coordinate table of FIG. 13. The corner coordinate table ofFIG. 13 may be stored in any desired memory of the inspection apparatus4.

Further, in this example, the master image generator 402 performs S801and S802 of FIG. 8 for each one of R, G, and B planes of the masterimage. Accordingly, the corner coordinate table of FIG. 13 is generatedfor each one of R, G, and B planes of the master image.

Referring back to FIG. 8, at S803, the master image generator 402 refersto the coordinate value of each one of the P pattern units stored in theP pattern availability table of FIG. 7, and the coordinate value of thecorner stored in the coordinate value table of FIG. 13, to select thecoordinate value that is located far most from the center of the masterimage, for each one of the areas A to D of the master image of FIG. 12,for each one of the color planes of R, G, and B. For the P pattern unitsstored in the P pattern availability table of FIG. 7, the master imagegenerator 402 only refers to the coordinate value of the P pattern unithaving the availability “Y” indicating that the P pattern unit can beused as a reference point.

More specifically, the master image generator 402 calculates, for eachone of the P pattern units of the P pattern availability table of FIG.7, a distance with respect to the center of the master image. The masterimage generator 402 further calculates a distance of the coordinatevalue of the corner for a specific area of the image with respect to thecenter of the master image. Using the calculated distance values, themaster image generator 402 selects the coordinate value of the P patternunit or the corner, which is located far most from the image center. Thereference point is selected for each one of the areas A to D of themaster image, based on the distance of the coordinate value of each ofthe P pattern units and the coordinate value of the corner. Thisselection process is repeated for each one of the area A to D, for eachone of color planes.

In alternative to selecting the reference point based on the distancewith respect to the image center, the master image generator 402 mayselect any one of the P pattern units and the corner, according to apriority level that is previously assigned to the P pattern unit. Forexample, the P pattern availability table of FIG. 7 may additionallystore priority information indicating a priority level assigned to eachone of the P pattern units. As the coordinate value is stored for eachone of the P pattern unit that is determined to be used as a candidatefor a reference point, the master image generator 402 may assign apriority level based on how far the coordinate value is located from theimage center. By referring to the priority level of the P patternavailability table of FIG. 7, the master image generator 402 is able toeasily specify the P pattern unit having the coordinate value that isfar most from the image center. The master image generator 402 comparesthe specified P pattern unit with the corner extracted from the masterimage to select one of the P pattern unit and the corner, which has thecoordinate value that is far most from the image center.

In case when the binary image is determined to be monochrome at S601 ofFIG. 6, the master image generator 402 does not perform S602 such thatthe P pattern availability table of FIG. 7 is not generated. In suchcase, the master image generator 402 selects the reference point basedon the coordinate value table of FIG. 13. More specifically, the materimage generator 402 selects the corner of the master image for each ofthe areas A to D, for each of color planes.

The master image generator 402 selects a reference point for each one ofthe areas A, B, C, and D of the master image to obtain the total of fourreference points in the master image, for each one of R, G, and B planesof the master image. The master image generator 402 further storesinformation indicating the selected reference point, such as informationused for specifying the coordinate value of the selected referencepoint. For example, as illustrated in FIG. 14, the master imagegenerator 402 generates a reference point selection result table, whichstores, for each one of the selected reference points, planeinformation, area information, coordinate information, and selectedpattern information in association with one another. The planeinformation indicates one of the R, G, and B planes of the master image,from which the reference point is extracted. The area informationindicates one of the A, B, C, and D areas of the master image, fromwhich the reference point is extracted. The coordinate informationindicates the coordinate value of the mater image at which the referencepoint is located. The selected pattern information indicatesidentification information for uniquely identifying the reference pointsuch as the pattern number or the coordinate number. The mater imagegenerator 402 inputs the mater image and the reference point selectionresult table of FIG. 14 to the inspection controller 404, and theoperation of FIG. 8 ends. The reference point selection result table ofFIG. 14 may be stored in any desired memory of the inspection apparatus4.

Referring now to FIG. 15, operation of comparing the master image andthe read image, performed by the comparator 405, is explained accordingto an example embodiment of the present invention.

At S1501, the comparator 405 obtains the read image from the read imageobtainer 401 under control of the inspection controller 404.

At S1502, the comparator 405 obtains the master image and the referencepoint selection result table of FIG. 14 from the master image generator402 under control of the inspection controller 404.

At S1503, the comparator 405 corrects pixel positions of the masterimage such that the pixel positions match between the master image andthe read image, based on the read image and the reference pointselection result table.

Referring now to FIG. 16, operation of correcting the master image,performed by the comparator 405 at S1503, is explained according to anexample embodiment of the present invention.

At S1601, the comparator 405 extracts a plurality of reference points inthe read image, which respectively correspond to the plurality ofselected references points in the master image, using the referencepoint selection result table of FIG. 14.

For example, referring to FIG. 14, the selected reference point for thearea A of the R plane of the master image is the P pattern unit that isidentified with the pattern number “#1, 1”, which is located at thecoordinate value indicated by the coordinate information of FIG. 14. Thecomparator 405 searches through the read image for an area including aplurality of pixels surrounding a pixel that is located at thecoordinate that corresponds to the specific coordinate of the referencepoint defined in the master image. The comparator 405 extracts acoordinate value of the pixel that corresponds to the reference point inthe master image, as the reference point in the read image.

The area of the read image that is subjected for searching may be set,depending on the estimated value of positional shift that may bedetected in the read image with respect to the master image. Theestimated value of positional shift may be previously determined, usingthe relationship between the value of positional shift and anaccumulated number of printed images that are sequentially printed bythe print processor 301. For example, the relationship between thepositional shift and the accumulated number of printed images may bestored in a memory of the inspection apparatus 4, such as the ROM 30(FIG. 2). Assuming that the positional shift of 10 pixels is estimated,the comparator 405 extracts an image including a plurality of pixelssurrounding the selected reference point, which extends from theselected reference point by 10 pixels in the directions of upward,downward, right, and left, as an image subjected for searching.Accordingly, the comparator 405 searches through the extracted image of20 dots by 20 dots for the selected reference point, using patternmatching, to extract the reference point in the read image thatcorresponds to the selected reference point in the master image.

In another example, referring to FIG. 14, the selected reference pointfor the area B of the R plane of the maser image is the corner of themaster image, which is identified with the “coordinate number #3” and islocated at the coordinate value specified by the coordinate informationof FIG. 14. In such case, the comparator 405 extracts the corner of thearea B of the read image, for example, by applying filtering asdescribed above referring to S801 to S802 of FIG. 8.

The above-described process of extracting the reference point in theread image, which corresponds to the selected reference point in themaster image, is repeated for each one of the selected reference points.

At S1602, the comparator 405 compares the difference between thereference pint in the read image and the reference point in the masterimage to obtain the positional shift in the read image with respect tothe master image. Based on the positional shift, the comparator 405corrects the pixel positions of the master image so as to match with thepixel positions of the read image. For example, the comparator 405calculates an enlargement ratio, which causes the master image to havethe size that matches the size of the read image.

At 1603, the comparator 405 re-samples the master image based on thecalculated enlargement ratio to generate the corrected master image,which matches the read image in pixel positions, and the operation ends.

Referring back to FIG. 15, the comparator 405 compares between the readimage and the corrected master image to generate and output adifferential image. More specifically, the comparator 405 obtains thedifference in pixel value between each pixel in the read image and eachpixel in the corrected master image, and generates a differential imagecontaining the pixels having the obtained difference value. Assumingthat the read image, i.e., the printed image is in good quality, thedifference between the read image and the corrected master image isexpected to have smaller values as the halftone value of each pixel isnearly the same for the read image and the corrected master image. Thisresults in each pixel of the differential image to have a small valuenear “0”. When the read image is not in good quality, the differencebetween the read image and the corrected master image is expected tohave larger values as the halftone value of each pixel is different forthe read image and the corrected master image. This results in eachpixel of the differential image to have a larger value.

The comparator 404 further compares the differential image, or thedifference value, with a predetermined threshold to determine whetherthe printed image is a defect image. For example, the comparator 405 mayset a threshold for each of R, G, and B planes, and compares thedifferential value with the threshold for each of R, G, and B planes.Alternatively, the comparator 404 may calculate the shift in brightness,hue, and saturation (“color shift”) based on the difference for each oneof R, G, and B planes, and compares the calculated color shift with athreshold to determine whether the read image is a defect image. Whenthe difference exceeds the threshold, the comparator 405 determines thatthe read image is a defect image. When the difference does not exceedthe threshold, the comparator 405 determines that the read image issufficiently in good quality. The inspection result obtained at S1505may be output to a user, for example, through the LCD 60 as a messageunder control of the inspection controller 404. Alternatively, theinspection result may be output in the form of a sound such as a beepsound only when the defect image is detected. After the inspectionresult is output, the operation of FIG. 15 ends.

When the inspection result of the comparator 405 indicates that the readimage is a defect image, the inspection controller 404 may send arequest for re-printing the image to the engine controller 2. Based onthis request, the engine controller 2 instructs the print engine 3 toprint the image on the recording sheet.

As described above, the inspection apparatus 4 determines whether the Ppattern added to the image can be effectively used as a reference pointfor detecting the position shift in the read image with respect to themaster image, or correcting the positional shift in the read image withrespect to the master image. When the P pattern cannot be effectivelyused, the inspection apparatus 4 uses the coordinate value of the cornerof the image, which is previously extracted, as the reference point.

For example, when the printed image to be processed is a monochromeimage, the P pattern is not added to the printed image. In such case,the inspection apparatus 4 uses a plurality of corners, which arerespectively set in the master image and the read image, as a referencepoint to detect the positional shift between the master image and theread image.

In another example, when the background color of the printed image to beprocessed is yellowish, the P pattern, which is yellow in color, can behardly detected. In such case, the inspection apparatus 4 may use acorner that is respectively set in the master image and the read image,as a reference point to detect the positional shift between the masterimage and the read image.

For example, when the Y plane of the binary image contains image data,the inspection apparatus 4 determines that the background color of theprinted image is yellowish. In another example, when the backgroundcolor of the recording sheet is yellowish, the inspection apparatus 4determines that the background color of the printed image is yellowish.

In the above-described example, the inspection apparatus 4 generates theP pattern availability table of FIG. 7 to store information regardingone or more P pattern units that are candidates of reference point.Further, the inspection apparatus 4 generates the corner coordinatetable of FIG. 13 to store information regarding the corners of themaster image that are extracted as candidates of reference point. Usingthe P pattern availability table of FIG. 7 and the corner coordinatetable of FIG. 13, the inspection apparatus 4 selects the reference pointhaving the coordinate value that is far most from the image center, foreach of the areas A to D, for each of the color planes. This is becausethe reference point that is close to the corner of the image may beeffectively used to detect the positional shift, as the positional shiftdue to shrinking of the printed image tends to be greater at the cornerof the image.

In alternative to generating the P pattern availability table of FIG. 7,the inspection apparatus 4 may only use the corner coordinate table ofFIG. 13 to use the corner of the image as a reference point. In suchcase, the P pattern does not need to be generated.

In this example illustrated in FIG. 1, it is assumed that the DFE 1, theengine controller 2, the print engine 3, and the inspection apparatus 4are each implemented by a separate apparatus. The image forming systemof FIG. 1 may be implemented in various other ways.

For example, as illustrated in FIG. 17A, the DFE 1, the enginecontroller 2, and the print engine 3 may be incorporated into oneapparatus functioning as an image forming apparatus such as a printer.In another example, as illustrated in FIG. 17B, the DFE 1, the enginecontroller 2, the print engine 3, and the inspection apparatus 4 may beincorporated into one apparatus functioning as an image formingapparatus such as a printer.

In the above-described example illustrated in FIG. 1, the DFE 1, theengine controller 2, the print engine 3, and the inspection apparatus 4are connected through a local interface such as a universal serial bus(USB) or a Peripheral Component Interconnect express (PCIe).Alternatively, the inspection apparatus 4 may be provided at a site thatis remotely located from the DFE 1, the engine controller 2, and theprint engine 3, as long as the inspection apparatus 4 is capable ofcommunicating with the DFE 1, the engine controller 2, and the printengine 3. For example, the inspection apparatus 4 may be implemented byapplication that provides the inspection control function to the imageforming system or the image forming apparatus through the network.

For example, as illustrated in FIG. 18, the image forming systemincluding the DFE 1, the engine controller 2, and the print engine 3,and the inspection apparatus 4 are connected through a public networksuch as the Internet. The engine controller 2 and the print engine 3transmit various data or information to the inspection apparatus 4through the network 5. The inspection apparatus 4 transmits aninspection result to the engine controller 2 through the network 5. Inthis manner, the inspection apparatus 4 does not have to be provided atthe user site such that the user is able to reduce the initial cost thatmay be otherwise required to implement the function of inspecting theprinted image.

The above-described operation of correcting the positional shift in theread image with respect to the master image may be performed in variousother ways. For example, in alternative to correcting the master image,the read image may be corrected so as to cause the pixel positions tomatch between the master image and the read image.

Further, in alternative to using the yellow dot pattern as a dot patternto be used for detecting the positional shift, any desired dot patternmay be used as long as the dot pattern is not perceptible to the humaneye. For example, the pattern may be generated using colorant that isnot visible under the natural light.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that within thescope of the appended claims, the disclosure of the present inventionmay be practiced otherwise than as specifically described herein.

With some embodiments of the present invention having thus beendescribed, it will be obvious that the same may be varied in many ways.Such variations are not to be regarded as a departure from the spiritand scope of the present invention, and all such modifications areintended to be included within the scope of the present invention.

For example, elements and/or features of different illustrativeembodiments may be combined with each other and/or substituted for eachother within the scope of this disclosure and appended claims.

Further, any of the above-described devices or units can be implementedas a hardware apparatus, such as a special-purpose circuit or device, oras a hardware/software combination, such as a processor executing asoftware program.

Further, as described above, any one of the above-described and othermethods of the present invention may be embodied in the form of acomputer program stored in any kind of storage medium. Examples ofstorage mediums include, but are not limited to, flexible disk, harddisk, optical discs, magneto-optical discs, magnetic tapes, nonvolatilememory cards, ROM (read-only-memory), etc.

Alternatively, any one of the above-described and other methods of thepresent invention may be implemented by ASIC, prepared byinterconnecting an appropriate network of conventional componentcircuits or by a combination thereof with one or more conventionalgeneral purpose microprocessors and/or signal processors programmedaccordingly.

In one example, the present invention may reside in an image inspectionapparatus, which inspects a read image that is read from a printed imageformed and output by an image forming apparatus. The image inspectionapparatus includes: inspection image generating means for obtainingimage data of the printed image that is used for forming the printedimage by the image forming apparatus and generating the inspection imageto be used for inspecting the printed image based on the image data; andimage inspecting means for comparing the inspection image and the readimage to inspect the read image. The inspection image generating meansextracts a corner of the inspection image and stores the extractedcorner as a reference point to be used for correcting pixel positions ofthe inspection image and the read image. The image inspecting meansextracts a corner of the read image, and sets the extracted corner as areference point to be used for correcting pixel positions of theinspection image and the read image. The image inspecting means correctsthe pixel positions of the inspection image and the read image based ondifference between the reference point in the inspection image and thereference point in the read image, and compares between the inspectionimage and the read image to inspect the read image.

For example, the inspection image generating means corresponds to themaster image generator 402, which may be implemented by the CPU 10 thatoperates in cooperation with the inspection control program and/or thespecialized device 80. The image inspecting means corresponds to thecomparator 404, which may be implemented by the CPU 10 that operates incooperation with the inspection control program and/or the specializeddevice 80.

In one example, the inspection image generating means extracts an edgeimage from the inspection image based on difference in pixel valuebetween adjacent pixels, and further extracts the corner from the edgeimage. The image inspecting means extracts an edge image from the readimage based on difference in pixel value between adjacent pixels, andfurther extracts the corner from the edge image.

When executing the corner, the inspection image generating means and theimage inspecting means each segment the image to be processed into aplurality of areas, and extracts at least one corner for each one of theplurality of areas of the image.

The inspection image generating means and the image inspection imageeach select one of reference point candidates that is located far mostfrom the image center as the reference point for each one of theplurality of areas of the image.

In the image inspection apparatus, the inspection image generating meansgenerates the inspection image to which a pattern is added. The patternbeing added is a pattern that is the added to the printed image formedby the image forming apparatus. The inspection image generating meansfurther determines whether the pattern can be used as a reference point,based on whether an area of the inspection image to which the pattern isadded contains image data to generate a determination result, and storesthe determination result in a memory. The inspection image generatingmeans selects one of the pattern that is determined to be used as thereference point and the corner extracted from the inspection image,which is located far most from the image center, as the reference point.

In one example, the present invention may reside in an image formingapparatus, which includes image forming means for forming and outputtingthe printed image, and image reading means for reading the printed imageformed on the recording sheet and output from the image forming means.The image forming means corresponds to the print processor 301. Theimage reading means corresponds to the reading device 302.

In one example, the present invention may reside in a method ofinspecting a read image that is read from a printed image formed andoutput by an image forming apparatus. The method includes: obtainingimage data of the printed image that is used for forming the printedimage by the image forming apparatus; generating the inspection image tobe used for inspecting the printed image; extracting a corner of theinspection image to store the extracted corner as a reference point tobe used for correcting pixel positions of the inspection image and theread image; obtaining the read image and extracting a corner of the readimage to store the extracted corner as a reference point to be used forcorrecting pixel positions of the inspection image and the read image;correcting the pixel positions of the inspection image and the readimage based on difference between the reference point in the inspectionimage and the reference point in the read image; and comparing betweenthe inspection image and the read image to inspect the read image.

In one example, the present invention may reside in an image formingsystem, which inspects a read image that is read from a printed imageformed and output by an image forming apparatus. The image formingsystem includes the image forming apparatus, an image reading apparatusthat reads the printed image to generate the read image, and an imageinspection apparatus that detects the read image. The image inspectionapparatus includes: inspection image generating means for obtainingimage data of the printed image that is used for forming the printedimage by the image forming apparatus and generating the inspection imageto be used for inspecting the printed image; and image inspecting meansfor comparing the inspection image and the read image to inspect theread image. The inspection image generating means extracts a corner ofthe inspection image and stores the extracted corner as a referencepoint to be used for correcting pixel positions of the inspection imageand the read image. The image inspecting means extracts a corner of theread image, and sets the extracted corner as a reference point to beused for correcting pixel positions of the inspection image and the readimage. The image inspecting means corrects the pixel positions of theinspection image and the read image based on difference between thereference point in the inspection image and the reference point in theread image, and compares between the inspection image and the read imageto inspect the read image.

In one example, the present invention may reside in an image inspectionapparatus, which inspects a read image that is read from a printed imageformed and output by an image forming apparatus. The image inspectionapparatus includes: inspection image generating means for obtainingimage data of the printed image that is used for forming the printedimage by the image forming apparatus and generating the inspection imageto be used for inspecting the printed image; and image inspecting meansfor comparing the inspection image and the read image to inspect theread image. The inspection image generating means extracts a corner ofthe inspection image and stores the extracted corner as a referencepoint to be used for correcting pixel positions of the inspection imageand the read image. When the image data of the printed image ischromatic, the inspection image generating means generate the inspectionimage to which a pattern is added. The pattern being added is a patternthat is added to the printed image formed by the image formingapparatus. The inspection image generating means further determineswhether the pattern can be used as a reference point, based on whetheran area of the inspection image to which the pattern is added containsimage data to generate a determination result, and stores thedetermination result in a memory. The inspection image generating meansselects one of the pattern that is determined to be used as thereference point and the corner extracted from the inspection image,which is located far most from the image center, as the reference point.The image inspection means extracts a reference point to be used forcorrecting pixel positions of the inspection image and the read image,which corresponds to the reference point in the inspection image. Theimage inspecting means corrects the pixel positions of the inspectionimage and the read image based on difference between the reference pointin the inspection image and the reference point in the read image, andcompares between the inspection image and the read image to inspect theread image.

In one example, the present invention may reside in a non-transitoryrecording medium storing a plurality of instructions which, whenexecuted by a processor, cause the processor to perform any one of theabove-described operation of inspecting a read image read from a printedimage formed by an image forming apparatus.

For example, the method includes: obtaining image data of the printedimage, which is used by the image forming apparatus to form the printedimage; generating a master image based on the image data of the printedimage, the master image being added with a plurality of units ofpattern; storing in a memory pattern location information indicating aspecific location in the master image to which a specific unit ofpattern is added, for each one of the plurality of units of pattern;extracting one or more corners of the master image; storing in thememory corner location information indicating a specific location in themaster image from which a specific corner is extracted, for each one ofthe one or more corners of the master image; selecting at least one ofthe plurality of units of pattern and the one or more corners of themaster image as a reference point to be used for detecting a positionalshift between the read image and the master image, using the patternlocation information and the corner location information; extracting areference point in the read image, which corresponds to the selectedreference point in the master image; correcting pixel positions of oneof the read image or the master image based on the positional shiftbetween the reference point in the read image and the reference point inthe master image to generate a corrected image; and inspecting the readimage based on difference between the corrected image and the other oneof the read image or the master image that is not corrected to generatean inspection result indicating whether the printed image sufficientlyreproduces the image data of the printed image.

In another example, the present invention may reside in an inspectionapparatus to inspect the read image, which includes: means for obtainingimage data of the printed image, which is used by the image formingapparatus to form the printed image; means for generating a master imagebased on the image data of the printed image, the master image beingadded with a plurality of units of pattern; means for storing patternlocation information indicating a specific location in the master imageto which a specific unit of pattern is added, for each one of theplurality of units of pattern; means for extracting one or more cornersof the master image; means for storing corner location informationindicating a specific location in the master image from which a specificcorner is extracted, for each one of the one or more corners of themaster image; means for selecting at least one of the plurality of unitsof pattern and the one or more corners of the master image as areference point to be used for detecting a positional shift between theread image and the master image, using the pattern location informationand the corner location information; means for extracting a referencepoint in the read image, which corresponds to the selected referencepoint in the master image; means for correcting pixel positions of oneof the read image or the master image based on the positional shiftbetween the reference point in the read image and the reference point inthe master image to generate a corrected image; and means for inspectingthe read image based on difference between the corrected image and theother one of the read image or the master image that is not corrected togenerate an inspection result indicating whether the printed imagesufficiently reproduces the image data of the printed image.

For example, the means for obtaining, the means for generating, and themeans for storing pattern location information, the means forextracting, and the means for storing corner location information, andthe means for selecting, correspond to the master image generator 402.The means for extracting a reference point in the read image, the meansfor correcting, and the means for inspecting correspond to thecomparator 405.

1-12. (canceled)
 13. An inspection apparatus to inspect a read imagefrom a printed image output from an image forming apparatus, theinspection apparatus comprising: a processor configured to, obtain imagedata of the printed image, used by the image forming apparatus to formthe printed image; generate a master image based on the obtained imagedata; extract one or more corners in the master image; extract one ormore reference points in the read image corresponding to the one or morecorners, respectively; correct pixel positions of one of the read imageand the master image based on a positional shift between the one or morereference points and the one or more corners to generate a correctedimage; and inspect the read image based on a difference between thecorrected image and the other one of the read image and the master imageto generate an inspection result indicating whether the printed imagesufficiently reproduces the image data of the printed image.
 14. Theinspection apparatus of claim 1, further comprising: a memory configuredto store corner location information in the master image from which theprocessor extracts each one of the one or more corners.
 15. Theinspection apparatus of claim 2, wherein the memory is furtherconfigured to store, for each one of the one or more corners in themaster image, identification information for identifying each one of theone or more corners, and the corner location information, in associationwith one another.
 16. The inspection apparatus of claim 3, wherein theprocessor is configured to select at least one of the one or morecorners having a greatest distance between a location specified by thecorner location information and a center of the master image.
 17. Theinspection apparatus of claim 1, wherein the processor is configured to,add a plurality of units of pattern to the master image, and determineto use the one or more corners to extract the one or more referencepoints, based on a determination of whether the positional shift isdetectable using the plurality of units of patterns in the master image.18. An image forming system, comprising: an image forming apparatusconfigured to output a printed image and cause an image reading deviceto read the printed image; and an inspection apparatus configured toinspect the read image, the inspection apparatus including, means forobtaining image data of the printed image used by the image formingapparatus to form the printed image; means for generating a master imagebased on the image data; means for extracting one or more referencepoints in the read image, corresponding to the one or more corners,respectively; means for correcting pixel positions of one of the readimage and the master image based on a positional shift between the oneor more reference points and the one or more corners to generate acorrected image; and means for inspecting the read image based on adifference between the corrected image and the other one of the readimage or the master image to generate an inspection result indicatingwhether the printed image sufficiently reproduces the image data of theprinted image.
 19. A non-transitory computer-readable medium including acomputer program product, the computer program product storing aplurality of instructions which, when executed by a processor, cause theprocessor to perform a method of inspecting a read image read from aprinted image output from an image forming apparatus, the methodcomprising: obtaining image data of the printed image, used by the imageforming apparatus to form the printed image; generating a master imagebased on the image data; extracting one or more reference points in theread image corresponding to the one or more corners, respectively;correcting pixel positions of one of the read image and the master imagebased on a positional shift between the one or more reference points andthe one or more corners to generate a corrected image; and inspectingthe read image based on a difference between the corrected image and theother one of the read image or the master image to generate aninspection result indicating whether the printed image sufficientlyreproduces the image data of the printed image.